Toby Keith vs REK, plagiarism/inspired by/coincidence?

Toby Keith:
Bullets hit the gun

Robert Earl Keen:
Road goes on forever

REK response to Toby by song:
Road goes on and on

It’s one thing for music/chord progression to be similar coincidentally (ie Steve Earle’s Dixieland and Charlie Robison’s John Orielly), but when the music is similar and the song’s story is similar as well, it’s a little weird. At what point does an homage turn into plagiarism? What’s the threshold for changing enough of the music or the lyrics to avoid such charges?

Interestingly, TK gave Jack Ingram a record deal on his label, and I always considered Ingram to be somewhat of an heir to REK in the Texas music genre, and a trailblazer for others like Pat Green et al., before Boland/Ragweed/Stony Larue hit the scene